Recent research on infants' understanding of the physical world indicates that infants share many of adults' beliefs about objects. For example, there is now substantial evidence that even young infants appreciate that objects cannot remain stable without support. The proposed research builds on these findings. It explores infants' ability to use their physical knowledge to make decisions about (a) which surfaces belong to the same object and (b) which objects belong to the same category. Segregation and categorization are both tasks that show clear knowledge effects in adults. Preliminary findings indicate that young infants can use their intuitions about support to segregate and categorize objects. The proposed experiments will attempt to extend these findings by considering other facets of infants' physical knowledge. In addition, the proposed experiments will begin to explore how infants come to integrate their physical knowledge with conflicting information, especially perceptual information, in grouping surfaces or objects. Preliminary results suggest that, in both segregation and categorization tasks, infants go through phases in which they attach greater importance to perceptual than to physical knowledge information. Experiments are planned. to test the generality of these phases, and to specify the experiences that enable infants to progress past these phases.